A variety of optical communications modules are used in optical networks for transmitting and receiving optical data signals over the networks. An optical communications module may be (1) an optical receiver module that has optical receiving capability, but not optical transmitting capability, (2) an optical transmitter module that has optical transmitting capability, but not optical receiving capability, or (3) an optical transceiver module that has both optical transmitting and optical receiving capability.
A typical optical transmitter module has a transmitter module housing, an electrical subassembly (ESA) disposed within the housing, and an optics system module secured to the transmitter module housing or to the ESA. The ESA of the transmitter module typically includes a module printed circuit board (PCB), a laser driver circuit mounted on the module PCB, at least one laser diode mounted on the module PCB, and various other electrical components mounted on the module PCB. The laser driver circuit outputs an electrical drive signal to each respective laser diode to cause the respective laser diode to be modulated. When the laser diode is modulated, it outputs optical signals that have power levels corresponding to logic 1s and logic 0s. The optics system module performs the function of directing the optical signals produced by each respective laser diode into the end of a respective transmit optical fiber. The ends of the transmit optical fibers are mechanically and optically coupled to the optics system module by some type of securing mechanism. The module PCB is often mounted on a higher-level system PCB and electrical interconnections are made between the two PCBs.
A typical optical receiver module has a receiver module housing, an ESA disposed within the housing, and an optics system module secured to the receiver module housing or to the ESA. The ESA includes a module PCB, at least one receive photodiode mounted on the module PCB, and various other electrical components mounted on the module PCB. The optics system of the optical receiver module directs an optical data signal that is output from the end of an optical fiber onto one of the respective photodiodes. The photodiode converts the incoming optical data signal into an electrical signal. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signal produced by the photodiode and outputs a corresponding amplified electrical signal, which is processed by other circuitry of the ESA to recover the data. The module PCB is often mounted on a higher-level system PCB and electrical interconnections are made between the two PCBs.
In optical transceiver modules, the receiver and transmitter ESAs are typically consolidated into a single ESA having a single module PCB on which the laser diodes, photodiodes and other electrical components of the receiver and transmitter are mounted. All of these components are housed within a transceiver module housing. A single optics system module is typically used in the optical transceiver module for coupling optical signals between the ends of the optical fibers and the respective photodiodes and laser diodes.
During the process of assembling a transmitter, receiver or transceiver module of the type described above, surface mount components of the ESA are first mounted on the module PCB with a standard surface mount technology (SMT) process. Typical surface mount components include resistors, capacitors, inductors, and clock and data recovery (CDR) circuitry. The ESA is then cleaned. After the SMT process has been performed and the ESA has been cleaned, a die attach process is performed to attach the laser diode driver circuit, the laser diode, the TIA, and the photodiode to the module PCB. During the die attach process, a machine vision system captures images of the components and of the PCB. Based on these images, the robotics system makes adjustments to the positions and orientations of the components to bring them into their proper positions and orientations on the PCB.
After the die attach process has been performed, wire bonding processes are performed to make all of the necessary electrical connections. After the die attach process has been performed, the ESA cannot be subjected to any additional solder reflow processes because solder reflow processes typically leave behind residue or debris such as solder flux and solder balls on the PCB and/or on the other components of the ESA. If not removed, the residue can result in performance problems. For example, solder flux can come into contact with bond wires resulting in electrical performance problems, or it can interfere with the optical pathways resulting in optical performance problems. Cleaning also is not possible at this stage because it can damage the wire bonds. In addition, it is difficult to wash out flux and debris in the tightly-spaced optical pathways. For these reasons, the optical communications module typically is not soldered to the system PCB, but rather, is typically configured to mechanically couple with a non-solder interface, such as a mechanical connector that is soldered to the system PCB. This non-solder interface adds to the cost of the system.
After the die attach and wire bonding processes have been performed, the optics system module is mounted onto the ESA. The optics system module is typically a molded plastic part having one or more optical coupling elements formed therein for coupling optical signals between the ends of the optical fibers and the respective optoelectronic components (i.e., the laser diodes and photodiodes). The optical coupling elements are typically refractive, diffractive or reflective optical elements (e.g., lenses, reflectors, diffractive gratings). The ends of the optical fibers are mechanically coupled to the optics system module via a direct coupling arrangement (e.g., butt coupling) or via an optical connector device that holds the ends of the optical fibers and mates with the module housing.
During the process of mounting the optics system module on the ESA, a precision pick and place system with a camera and a computer running pattern recognition software is used to recognize the positions of the optoelectronic component and the optics system module. After a few modules are made, they are often taken off line for measurement of alignment accuracy with a microscope system. Adjustments are made to the pick and place system according to the microscope measurement to ensure that the optical elements of the optics system module are in accurate alignment with the respective optoelectronic components of the ESA. These off line measurements and adjustments are performed periodically for tight process control.
One of the difficulties associated with the alignment process described above is that it is difficult to measure alignment accurately with a conventional optics system module. The optical axis defined by the line of the optoelectronic element, the center of the lens, and the image of the optoelectronic element needs to be in perfect alignment with the optical axis of the measurement microscope. The mechanical structure used to hold the fiber needs to constrain the fiber end face at exactly the same point at which the image of the optoelectronic element is focused.
In arrangements where the fiber ends are secured directly to the optical ports, the optical ports are typically round and have slightly larger diameters than the diameters of the fibers to ensure that the fiber can be inserted into the port and later fixedly secured to the port by epoxy. In such a design, there is an uncertainty, or error, due to the fiber being able to move around in the port. Alternatively, the optical port can have a slightly smaller diameter than the diameter of the fiber. In the latter case, the ends of the optical fibers, which are also round, are pressed into the respective ports until the end faces of the optical fibers are in abutment with stops disposed inside of the respective ports. In such arrangements, the fiber is said to be press fit into the port. The interference between the fiber and the material of which the port is made holds the fiber in place.
In order for this round/round interface between the fiber ends and the respective optical ports to be effective, a relatively large force needs to be exerted on the fiber to push the fiber into the port, which can damage the fiber. Also, because of the tight fit at this interface, it is possible that an air gap will exist between the fiber end face and the stop disposed inside of the port, which can lead to the occurrence of Fresnel losses at the interface. Refractive index matching (RIM) epoxy cannot be used at this interface to prevent such Fresnel losses from occurring. Typically, there is surface roughness at the end face of the fiber, both in the case of cleaved glass fiber and in the case of cut and polished plastic fiber. If RIM epoxy could be used in such arrangements, it would help to minimize optical loss by filling in the roughness. Hence, the lack of RIM epoxy at this interface introduces an extra loss due to the imperfection of the fiber end face.
A need exists for an optical communications module that can withstand solder reflow process so that it can be mounted via an SMT process to a system PCB. A need also exists for an optics system module that is more easily aligned with the optical communications module during the process of mounting the optics system module on the optical communications module, and that can eliminate or reduce the occurrence of Fresnel losses and losses due to imperfect fiber end faces at the interfaces between the fiber end faces and the optical ports of the optics system module.